perhaps you'll explain why the D800 score much better for high ISO than the 6D.

Nikon D800E 2979 ISO Low-light Score

Canon 6D 2340 ISO Low-light Score

Simple. The D800E has a higher quantum efficiency, which affects the amount of photon noise, which is dominant at middle gray at these ISO exposure indices, but the 6D has less read noise at high ISOs, which is not reflected in these ratings, but affect the amount of noise in the shadows, and at even much higher ISOs, where it is most critical.